In December 1978, the Federal Aviation Administration (FAA) amended their Fatigue Evaluation requirements for Transport Category Airplanes to include a damage tolerance philosophy. Prior to this time FAR 25.571, Fatigue Evaluation of Flight Apparatus, included an option to design to either “Fail-Safe” or “Safe-Life” principles. Manufacturers generally adopted the Fail-Safe option with the exception of a few components. In satisfying the regulations according to the Fail-Safe option, apparatus is designed to be redundant, so that catastrophic failure will not result after fatigue failure or obvious partial failure of a single principal structural element. However, the Fail-Safe approach does not include a disciplined engineering evaluation of crack growth and residual strength characteristics of each principal structural element using fracture mechanics technology necessary to specify inspection methods, thresholds and frequency that would detect damage prior to catastrophic failure.
Thus, in December 1978 the FAA released amendment 45 to FAR 25.571 requiring that new apparatus be designed to “Damage Tolerant” principles unless it could be shown that this approach would be impractical whereupon a “Safe-Life” option could be used. In May 198, an advisory circular AC-91.56 was issued to provide guidance material for the issue of supplemental Inspection Documents (SIDs) for existing Large Transport category Airplanes. Thus for both new designs and existing aircraft it is expected that engineering evaluation of the apparatus under typical load environmental spectra must show that catastrophic failure due to fatigue, accidental damage or corrosion will be avoided throughout the operational life of the aircraft. Since then, inspection programs for both new designs and existing older aircraft have been put in place based on a damage tolerance philosophy.
The above-mentioned inspection program of structural parts became part of the scheduled maintenance of the aircraft. From the airline perspective, it is desired to minimize the out-of-service maintenance, the cost of the inspection type and the labour involved in the activity. In other words fast inspection and long inspection intervals are desired. From the manufacturer's perspective, it needs to comply with airworthiness requirements while designing the part, accounting for uncertainties in the material characteristics, manufacturing processes, operational life of the aircraft and finally provide a cost effective design.
The challenge to comply with the Damage Tolerant philosophy while designing a cost effective part is increased if the most critical locations for future fatigue damage are in areas which are difficult to access and inspect. This can add a significant cost if inspection access needs to be provided through disassembly of parts, and some damage (such as crack initiation) can occur in this disassembly process. Disassembly may also be impractical if parts are bonded or co-cured. Therefore, every reasonable effort is made at the design stage to ensure inspectability of all structural parts and to quantify them under the damage tolerance provisions. In those cases where inaccessible and blind areas exist and suitable damage tolerance cannot practically be provided to allow for extension of a safe damage into detectable areas, the apparatus is shown to comply with the fatigue (“Safe-Life”) requirements in order to ensure its continued airworthiness.